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A graded composition superlattice structure is proposed by combining simulation with experimentation. The structural factors affecting graded symmetric superlattices and graded asymmetric superlattices and their action modes are simulated and analyzed. A Mg-doped graded symmetric superlattice structure with high Al content, excellent structural quality, good surface morphology and excellent electrical properties was grown by MOCVD equipment. The AlxGa1−xN superlattice with Al composition of 0.7 in the barrier exhibits a hole concentration of approximately 5 × 1015 cm−3 and a resistivity of 66 Ω·cm. [ABSTRACT FROM AUTHOR]

A graded composition superlattice structure is proposed by combining simulation with experimentation. The structural factors affecting graded symmetric superlattices and graded asymmetric superlattices and their action modes are simulated and analyzed. A Mg-doped graded symmetric superlattice structure with high Al content, excellent structural quality, good surface morphology and excellent electrical properties was grown by MOCVD equipment. The AlxGa1−xN superlattice with Al composition of 0.7 in the barrier exhibits a hole concentration of approximately 5 × 1015 cm−3 and a resistivity of 66 Ω·cm.

TiO 2 doping content affects the critical temperature (T c) and the variations of local structure on Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ. The (Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ) 1-x (TiO 2) x samples were fabricated with the solid-state reaction process, where x = 0, 0.002, 0.004, 0.006, 0.008, and 0.01. The T c values of the samples were obtained by measuring resistivity versus temperature, indicating that T c gradually decreased with increasing doping content (x). To explain the obtained degradation of T c , carrier properties, role of interlayer coupling, and local structure were systematically investigated using Azlamozov–Larkin theory and its Lawrence–Doniach modification for strong anisotropic superconductors. The calculation of excess conductivity at the mean field region showed that the c-axis coherence length (ξ c) and the effective inter-layering spacing (d) increased with increasing doping content. X-ray diffraction patterns also showed that the bond distances increased with increasing TiO 2 content. The copper valence (V) and carrier concentration (p) of the samples were determined by analyzing the Cu L 2,3 -edge X-ray absorption near edge structure spectra. The values of V and p showed the same trend of decreasing with increasing x. A close correlation between the changes in local structure parameters and degradation of T c of (Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ) 1-x (TiO 2) x was then probably concluded. [ABSTRACT FROM AUTHOR]

In this paper, we present the dependence of source resistance sensibility on the gate bias effect in a High Electron Mobility Transistor (HEMT) using the Drift-Diffus (D-D) model with the SILVACO Technology Computer-Aided Design (TCAD) tool. The obtained results show that the increases of gate bias effect on substrate lead to decreasing the source resistance of the simulated device. The reported increase in the effect of gate induces the increases of transferred holes concentration towards the source region and which induce the decreases of source resistance. The decrease of source resistance can also be made by reducing the buffer thickness which leads to an increase in the gate effect on the substrate. The source resistance value is influenced by the Drain-Induced Barrier Lowering (DIBL) effect where the rate of decreasing the source resistance will be decreasing consequently to increase the drain bias. The reduction of the source resistance induces the increase of device sensibility for lows values of current.

In this paper, we present the dependence of source resistance sensibility on the gate bias effect in a High Electron Mobility Transistor (HEMT) using the Drift-Diffus (D-D) model with the SILVACO Technology Computer-Aided Design (TCAD) tool. The obtained results show that the increases of gate bias effect on substrate lead to decreasing the source resistance of the simulated device. The reported increase in the effect of gate induces the increases of transferred holes concentration towards the source region and which induce the decreases of source resistance. The decrease of source resistance can also be made by reducing the buffer thickness which leads to an increase in the gate effect on the substrate. The source resistance value is influenced by the Drain-Induced Barrier Lowering (DIBL) effect where the rate of decreasing the source resistance will be decreasing consequently to increase the drain bias. The reduction of the source resistance induces the increase of device sensibility for lows values of current. [ABSTRACT FROM AUTHOR]

In this letter, novel heterostructure p-i-n GaN avalanche photodiodes (APDs) with p-type In0.05Ga0.95N layer are proposed and analyzed through Silvaco Atlas simulations. The introduction of the In0.05Ga0.95N layer generates a negative polarization charge at the p-In0.05Ga0.95N/i-GaN heterojunction interface via the piezoelectric polarization effect. Three times higher hole concentration in the p-In0.05Ga0.95N layer is obtained due to the smaller activation energy of the Mg dopant. Induced polarization charge and increased hole concentration work together to reduce the voltage drop in the p-In0.05Ga0.95N layer and enhance the electric field intensity in the i-GaN layer. The calculated results show that the heterostructure APD demonstrates a reduced operating voltage of more than 26 V and an improved multiplication gain by an order of magnitude in comparison to the conventional one.

In this work, a close correlation between variations of critical temperature (T c) and the hole concentration of (Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ) 1-x (Fe 3 O 4) x systems was studied. The (Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ) 1-x (Fe 3 O 4) x samples were fabricated using the solid-state reaction method, where x ranged from 0 to 0.2. The T c values of the samples deduced from magnetization versus temperature measurement gradually decreased with increasing the Fe 3 O 4 content (x). To investigate a possible reason for the observed decrease in the values of T c in the samples, the valence state of copper (V) and the hole concentration (p) in all samples were examined by analyzing the Cu K-edge and Cu L 2,3 -edge X-ray absorption near edge structure (XANES) spectra. The values of V and p monotonously decreased with the increase in Fe 3 O 4 doping content (x) and agreed with the behavior of T c. The existence of Fe3+ was confirmed by analyzing Fe L 2,3 -edge XANES. Hence, Fe3+ ions possibly entered the lattice structure of the samples and filled the holes in CuO 2 planes. The degradation of superconductivity in Fe 3 O 4 doped samples was then explained. [ABSTRACT FROM AUTHOR]

This review is on the effects of elemental substitutions at the Tl-site of TlSr 2 CaCu 2 O 7 (Tl-1212) superconductor. The objective of this paper was to determine the elements that enhance or suppress the superconducting transition temperature, T c of (Tl 1- x M x)Sr 2 CaCu 2 O 7 phase. The elements included in this review were M = Pb, Bi, Cr, V, Re, Zr, In, Er, Gd, Na, K, Rb and Se. The relations between transition temperature and the ionic radius, coordination number, valence state and optimization of the hole concentration of the selected elements were studied. The preparation methods, sintering temperature, Tl-1212 phase and structural stability were also discussed. All alkali metals (K, Na and Rb) suppressed the transition temperature of the Tl-1212 phase. The transition temperature generally decreased linearly with ionic radius except for Cr which showed the highest T c although its ionic radius is smaller. [ABSTRACT FROM AUTHOR]

In this letter, novel heterostructure p-i-n GaN avalanche photodiodes (APDs) with p-type In0.05Ga0.95N layer are proposed and analyzed through Silvaco Atlas simulations. The introduction of the In0.05Ga0.95N layer generates a negative polarization charge at the p-In0.05Ga0.95N/i-GaN heterojunction interface via the piezoelectric polarization effect. Three times higher hole concentration in the p-In0.05Ga0.95N layer is obtained due to the smaller activation energy of the Mg dopant. Induced polarization charge and increased hole concentration work together to reduce the voltage drop in the p-In0.05Ga0.95N layer and enhance the electric field intensity in the i-GaN layer. The calculated results show that the heterostructure APD demonstrates a reduced operating voltage of more than 26 V and an improved multiplication gain by an order of magnitude in comparison to the conventional one. [ABSTRACT FROM AUTHOR]

We present a distinct approach to enhance the performance of physically doped tunnel field-effect transistors (TFETs) based on creation of a layer of positive charge at the semiconductor-insulator interface in the source region. Formation of such a hole layer resolves the issue related to material solubility and improves direct-current (DC) as well as high-frequency figures of merit. To implement this approach, a typical P+-I-N+-type physically doped TFET structure is considered. Furthermore, a metal electrode with workfunction of 4.53 eV is placed over the heavily doped P+ source region with a negative supply voltage. The negative voltage at the source electrode attracts holes from the source region and creates a hole layer just below the semiconductor-insulator interface. This phenomenon makes the source-channel junction abrupt and reduces the tunneling barrier width, resulting in higher tunneling generation rate of charge carriers at the source-channel junction. Thus, the proposed device shows 100-fold increased ON-state current and a threshold voltage reduction of 300 mV. Analog/radiofrequency (RF) parameters are also greatly improved compared with the conventional device. Furthermore, optimization of the spacer length (LSG), i.e., the gap between the source and gate electrode, and application of a negative voltage (-VSE) at the source electrode (SE), were applied to achieve the optimum performance. Moreover, device linearity was also analyzed in a comparative manner. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of Nd1-xPrxBa2Cu3O7-δ with 0≤x≤0.30 were made by standard solid state methods. The transport and superconducting properties were studied by the resistivity and the thermoelectric power measurements as a function of temperature and doping concentration. The resistance and the thermoelectric power were increased by increasing doping concentration. The pseudogap temperature was measured from downturn deviation in a linear dependence of resistance as a function of temperature. The resitivity, the thermoelectric power, the critical temperature, and the pseudogap temperature results suggested that the hole concentration in CuO2 planes was decreased by increasing Pr doping concentration. The resistivity and the thermoelectric power results were analyzed by the bipolaron and the phenomenological narrow band model, respectively. A good agreement between models and data was obtained. On the basis of these models, it was inferred that the localization tendency is one of the main reasons for decreasing the hole concentration in the CuO2 plane, which results in the strong depression of the superconductivity.

The effect of CpMg/Ga ratio on the properties of p-GaN was explored by scanning Hall probe, photoluminescence (PL), and atomic force microscopy measurement. It was found that p-GaN has an optimal doping concentration under 2% CpMg/Ga ratio, and higher or lower doping concentration is not beneficial to the conductivity. Hole concentration under the optimum condition is 4.2 × 10 cm at room temperature. If the CpMg/Ga ratio exceeds the optimum value of 2%, surface morphology and electrical conduction properties become poor, and blue emission at 440 nm, considered deep donor-to-acceptor pair transitions in the PL spectra, are dominant. The decrease in electrical properties indicates the existence of compensating donors because the hole concentration decreases at such high CpMg/Ga ratio. The obtained results indicate that Mg is not incorporated in the exact acceptor site under a heavy doping condition, but acts as a deep donor, instead. [ABSTRACT FROM AUTHOR]

Orthorhombic Cu 3 SnS 4 nanocrystals (NCs) possessing obvious localized surface plasmon resonances (LSPR) in the near-infrared region (NIR) frequencies are prepared. In Cu 3 SnS 4 NCs, valence state of Cu remains close to +1, indicating Cu + occupies positions originally belong to Cu 2+ in a similar way as Cu + replaces Zn 2+ to form Cu Zn antisites in Cu 2 ZnSnS 4 . The increase of Cu/Sn ratio results in slightly red-shifted and weakened NIR absorption. Moreover, LSPR is also verified in wurtzite Cu 2 SnS 3 NCs, although the observed peak maximum is red-shifted comparing to the LSPR band of Cu 3 SnS 4 NCs. But the calculated free carrier concentration for Cu 2 SnS 3 by Drude model is on the same order of magnitude with Cu 3 SnS 4 in which sulfur vacancies acting as compensation centers could reduce hole density to some extent. Whereas Cu 2 ZnSnS 4 presents almost no absorption due to its relatively lower hole concentration. [ABSTRACT FROM AUTHOR]

[Display omitted] • α -BaCu 2 Se 2 is a wide-band-gap semiconductor with orthorhombic structure. • Cu vacancy engineering was applied in Zintl α -BaCu 2 Se 2 aiming to improve the thermoelectric properties. • The Cu vacancy can realize the temperature-dependent hole concentration to the optimal one. • Cu vacancy engineering signifies the enlargement of average zT in a wide temperature range. Vacancy engineering offers an alternative route to aliovalent and interstitial doping for optimization of the carrier concentration in thermoelectric materials. For the wide-bandgap semiconductor BaCu 2 Se 2 , the Cu vacancy is dynamically stable; this feature can be rationally manipulated to maximize the thermoelectric figure of merit zT. In this work, we show that at room temperature, Cu-deficient BaCu 2− x Se 2 samples exhibit increased hole effective mass and mobility, attributed to the energy band modulation, which are favorable for improved electrical transport properties. More importantly, the defect energy level resulting from the Cu vacancies continually contributes holes at high temperature, thereby allowing the hole concentration to approach an optimal concentration. This effect leads to an increase of the power factors over a wide temperature range. The artificial reduction of the Cu content in BaCu 2 Se 2 results in the strengthened point-defect scattering, suppressing the lattice thermal conductivity. This strategy allows simultaneous optimization of the electrical and thermal transport properties, with a thermoelectric figure of merit zT = 1.08 achieved for BaCu 1.94 Se 2 at 823 K, which is 38% higher than that of stoichiometric BaCu 2 Se 2. Within the measured temperature range, the average zT value for BaCu 1.94 Se 2 is 0.494, which is 52.9% higher than that of BaCu 2 Se 2. [ABSTRACT FROM AUTHOR]

Abstract: Samples of Eu1−x Pr x Ba2Cu3O7−δ with compositions x=0.0, 0.1 and 0.3 were prepared by the solid state reaction route. The structural, electrical and microstructural properties were investigated by X-ray diffraction, Physical Property Measurement System (PPMS) and scanning electron microscopy (SEM) techniques respectively. The lattice parameters, oxygen deficiency, hole concentration in the CuO2 plane, average charge on Cu–O plane were evaluated from X-ray diffraction data. From XRD data, it is observed that the lattice parameters and oxygen deficiency increase with increasing Pr concentration. It was also observed that the hole concentration in CuO2 plane and average charge on Cu–O plane decrease with increasing Pr concentration. The electrical resistivity of all samples was measured in the temperature range 300K to 4K. It was found that the normalized transition temperature decreases with increasing Pr concentration. This may be due to reduction in hole concentration or average charge on Cu–O plane. From scanning electron microscopy images, it was observed that grain size increases with increasing Pr concentration. This increase may be the possible cause for suppression of T c. [Copyright &y& Elsevier]

Strongly overdoped samples of the three-CuO2-plane copper-oxide superconductor, CuBa2Ca2Cu3O8+z or Cu-1223, were obtained through high-pressure synthesis and post-annealed to various hole-doping levels so as to have the value of Tc range from 65 to 118 K. A concomitant decrease in the average valence of copper from ∼2.20 to ∼2.05 was evidenced by means of wet-chemical and thermogravimetric analyses and Cu L-edge X-ray absorption near-edge structure (XANES) spectroscopy. The valence value as low as ∼2.05 that corresponds to the highest Tc (=118 K) may be understood by taking into account multiple ways for holes to be distributed among the different Cu–O layers. In terms of actual chemical composition of the Cu-1223 phase, both Cu L-edge and O K-edge XANES results suggest that some portion of charge-reservoir copper atoms may have been replaced by CO, i.e., (Cu1−xCx)Ba2Ca2Cu3O8+x+z. The variation range of excess oxygen was estimated at Δz≈0.3. [Copyright &y& Elsevier]

We report our studies on the photo doping effect in YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) grain-boundary weak links and partially oxygen-depleted (T/sub c,mid/ /spl ap/52 K) thin films, We focus our attention on the erasable process, in which persistent photoconductivity (PPC) introduced by light at low temperatures relaxes back above 260 K. In the tested 2-/spl mu/m-wide step-edge grain-boundary junctions, PPC manifested itself as above 30% increase of the critical current, while the illuminated films exhibited up to 2.5 K enhancement of T/sub c/'s. In the case of films, we also observed upon illumination a continuous decrease (no saturation effect) of both the longitudinal and transversal (Hall) resistivities. In-situ Hall effect band model, PPC at low temperatures results in the increase of both the hole mobility and concentration, while at temperatures above 250 K, the mobility is reduced after prolonged illumination, whereas the carrier concentration is enhanced even more than allow temperatures. We have also demonstrated that the T/sub c/ enhancement due to photodoping is a function of the carrier concentration rather than the mobility, Our experimental results indicate that the physical origin of PPC is same in both YBCO grain-boundary junctions and partially oxygen-depleted films, and can be understood as the coexistence of photoassisted oxygen ordering and photoinduced charge transfer. [ABSTRACT FROM PUBLISHER]

Sb anion-substituted gallium nitride films were fabricated by radio frequency reactive sputtering with single Sb-containing cermet targets with different Sb contents under Ar/N2 atmosphere. n-type GaN films with electron concentration of (1.40 ± 0.1) × 1017 cm−3 inverted to p-type Sb-GaN with hole concentration of (5.50 ± 0.3) × 1017 cm−3. The bandgap energy of Sb anion-added Sb-GaN films decreased from 3.20 to 2.72 eV with increasing Sb concentration. The formation of p-type Sb-GaN is attributed to the formation of Ga vacancy at higher Sb concentration. The coexistence of Sb at the Ga cation site and N anion site is an interesting and important result, as GaNSb had been well developed for highly mismatched alloys. The hetero-junction with p-type Sb-GaN/n-Si diodes was all formed by radio frequency (RF) reactive sputtering technology. The electrical characteristics of Sb-GaN diode devices were investigated from −20 to 20 V at room temperature (RT). [ABSTRACT FROM AUTHOR]

Nitride-bass light-emitting diodes (LEDs) with Mg-doped In[sub0.23]Ga[sub0.77]N capping layers were successfully fabricated. Compared to Mg-doped GaN layers, it was found that we could achieve a much larger hole concentration from Mg-doped In[sub0.23]Ga[sub0.77]N layers. It was also found that we could reduce the 20 mA operation voltage from 3.78 to 3.37 V by introducing a 5-nm-thick In[sub0.23]Ga[sub0.77]N layer on top of the p-GaN layer. Furthermore, it was found that output intensity of LEDs with In[sub0.23]Ga[sub0.77]N capping layer was much larger, particularly at elevated temperatures. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of Nd1-xPrxBa2Cu3O7-δ with 0 ≤ x ≤ 0.30 were made by standard solid state methods. The transport and superconducting properties were studied by the resistivity and the thermoelectric power measurements as a function of temperature and doping concentration. The resistance and the thermoelectric power were increased by increasing doping concentration. The pseudogap temperature was measured from downturn deviation in a linear dependence of resistance as a function of temperature. The resistivity, the thermoelectric power, the critical temperature, and the pseudogap temperature results suggested that the hole concentration in CuO2 planes was decreased by increasing Pr doping concentration. The resistivity and the thermoelectric power results were analyzed by the bipolaron and the phenomenological narrow band model, respectively. A good agreement between models and data was obtained. On the basis of these models, it was inferred that the localization tendency is one of the main reasons for decreasing the hole concentration in the CuO2 plane, which results in the strong depression of the superconductivity. [ABSTRACT FROM AUTHOR]